The present invention relates to the field of illumination for electronic displays. More particularly, the invention relates to an improved apparatus for providing high brightness backlights for electronic displays such as liquid crystal displays (LCDs).
Cold cathode fluorescent lamps (CCFLs) provide backlight illumination for electronic displays such as LCDs. Multiple CCFL tubes are installed behind an LCD to increase the brightness and corresponding visibility of the illuminated display. CCFLs differ from conventional fluorescent light tubes. Conventional tubes warm up gases within the tube to "strike" or illuminate at a lower voltage than otherwise possible at ambient temperature. CCFLs do not use heater elements, and the internal gas ionization process for CCFLs is initiated by high voltage. To accomplish this high voltage, a separate capacitor is associated with each CCFL tube in series. A high voltage power supply such as an inverter provides up to 2000V AC to each capacitor at a frequency between 25 and 60 KHz.
Before each tube strikes to provide illumination there is infinite impedance across the tubes so that no current flows. Accordingly there is no voltage drop across the capacitors and the voltage across the tubes is the full inverter output voltage. When each tube strikes current flows therethrough and illuminates the region proximate to the tube. The value of the capacitor is that when the appropriate current flows the voltage drop reduces the tube voltage to the proper running value. For 300 mm long tubes, the operating voltage approximates 700V.
In multiple tube displays, a separate hot wire runs from the inverter to each capacitor and a common return wire completes the multiple circuits. For a six tube display, six hot wires and one return wire are required to provide the requisite voltage to each capacitor. Multiple wires increase manufacturing difficulty and expense and limits the ability to shrink the overall system size. Each wire increases electrical loss due to high frequency radiation. In addition, each additional wire reduces the operating reliability of the system by increasing the number of components subject to failure.
For larger displays and for uses requiring bright displays, the quantity and density of tubes increases the number of wires leading from the inverter to the tubes. If the density of tubes could be increased for a display, a slimmer backlight would be possible and less diffuser material would be required to blend the produced light. The average current consumed by each tube in a dense configuration could be less to achieve the same overall illumination, thereby extending the useful tube life for each individual tube. However merely adding more tubes would result in the disadvantages associated with multiple lead wires.
A need, therefore, exists for an improved CCFL system capable of providing high brightness illumination for electronic displays. The system should be efficient to manufacture, should increase the density of tubes associated with an electronic display, and should be reliable.